This invention relates generally to liquid-junction semiconductor devices for use as photocells and in particular to such devices for use as solar cells for producing electrical energy from solar energy.
Concern over the continued availability as well as the continually escalating cost of fossil fuel energy sources has sustained high interest in the development of alternative energy sources, including solar power, which can be used to generate electricity. The devices most often considered for conversion of solar power into electricity are semiconductor devices, commonly called solar cells, which collect light, and generate photocurrent, in approximate proportion to the area of the photosensitive junction. This photosensitive junction must, therefore, be large to generate a useful current. The cost of manufacturing such devices depends in part on the area of the photosensitive junction and is presently too high to permit commercial exploitation of solar cells for other than limited and specialized applications.
Considerable effort has been devoted to finding ways to reduce the cost of semiconductor solar cell devices. A portion of this effort has been directed, as in U.S. Pat. No. 3,953,876, issued Apr. 27, 1976, to devices in wich the semiconductor material is deposited as a polycrystalline thin film on an expensive substrate rather than grown by the costly single crystal techniques used in earlier solar cells. A different approach that has generated enthusiasm is the liquid-junction semiconductor solar cells. The active part of these cells is a junction formed at a semiconductor-liquid interface. Because the junction forms spontaneously at the liquid-solid interface, the device promises to be less costly to manufacture as relatively costly epitaxy or diffusion procedures required for the single crystal or polycrystalline devices mentioned above are not needed to form the junction.
Four obstacles must be surmounted, however, before such cells can be exploited commercially. First, liquid-junction semiconductors are often not photochemically stable because photoexcitation produces electrons or electron holes at the semiconductor surface which may react with the semiconductor, causing corrosion of the semiconductor surface. This corrosion proceeds in a manner that degrades the desired characteristics of the semiconductor surface and is commonly manifested by decay of the photocurrent from the cell with operating time. An example of such a reaction with a CdS electrode, for example, is CdS(s)+2h.sup.+ .fwdarw.S.sup.0 (s)+Cd.sup.2+ (solvated) leading to the formation of a sulfur layer at the junction interface. One approach to solving this problem involves the use of, for example, a polysulfide-sulfide redox couple type of solution. Since the corrosion reaction CdS(s)+2h.sup.+ .fwdarw.S.sup.0 (s)+Cd.sup.2+ (solvated) proceeds at a higher electrode potential than the reaction S.sup.2- (solvated).fwdarw.S.sup.0 +2e.sup.-, the sulfur-polysulfide couple consumes the electron holes responsible for the corrosion reaction before the potential for the corrosion reaction is reached. A second approach to resolving this problem is to use a material which has a corrosion reaction potential so high as to in effect be corrosion resistant. Such materials are, for example, certain transition metal oxide compounds. A specific example is titanium dioxide.
Secondly, the cost of single crystal semiconductor electrodes is too high for commercial success. Several approaches have been tried to reduce the cost of single crystal semiconductor, especially chalcogenide, electrodes. One approach involves the electrolytic co-deposition of the electrode materials, e.g., cadmium and selenium, on an inert substrate. Another approach involves the anodization of a cadmium or bismuth substrate to form a chalcogenide semiconductor. These methods, however, do not produce materials which are cost competitive in the market place.
Thirdly, the band gap of the photoelectrodes must be closely attuned to the major energy portion of the solar spectrum, i.e., approximately 1.4 eV. This band gap is necessary not only to produce maximum power per surface area thereby increasing the output of a given cell but also to decrease the area of the liquid-solid junction needed and thereby lower the cost of the installation per unit of energy produced.
Finally, the liquid-junction semiconductor photocell needs to be one which is environmentally sound. Thus, while materials such as cadmium and selenium may produce potentially useful power outputs when used in solar cells, they are themselves highly toxic materials. Thus, they are not only environmentally harmful in use but also difficult and expensive to manufacture due to the necessary environmental considerations needed in the manufacturing processes of these materials.